In a typical Fluid Catalytic Cracking (FCC) refinery, the FCC absorber off-gases contributes to approximately one-third of the refinery fuel gas production. The absorber off-gases typically contain 15 to 20 mol % ethylene and ethane. These components are valuable products. Mixtures of ethylene and ethane can be good feed sources for ethane cracking facility for ethylene production. Currently most ethylene and ethane in FCC off-gas is burned instead of recovered. Removal of ethylene and heavier components has a very large economic benefit.
Conventionally, the primary method of recovering the FCC fuel gas ethylene and ethane is through a cold box where the FCC absorber off-gas is cooled to very low temperatures of around −120° C. and then light ends are separated out through a series of distillation columns. However, the cold boxes are capitally intensive. Further, the cold boxes also face the problem of potential explosions from trace NO in the FCC fuel gas. NO in the FCC absorber off-gas condenses within the cold box, followed by subsequent reaction with oxygen to form heavier NOX compounds that subsequently react with dienes to form gums, which may explode when the cold box is warmed up. This condensation of NO is caused by the extremely low temperatures found in a cold box in the range of −100° C. to −120° C. This safety problem discourages companies from mixing FCC fuel gas within existing product recovery systems even when there is extra capacity in these recovery section.
U.S. Pat. No. 5,245,099 describes pressure swing adsorption (PSA) recovery of ethylene from FCC fuel gas. However, since the PSA cannot totally purify the ethylene and substantial methane (around 15%) is still present, this stream would require further purification at the ethylene cracker demethanizer column if it was sent as is. The ethylene cracker demethanizer column runs very cold (−100° C.) and is integrated with the cold box. This would mean that small amounts of NO present in the feed would get very cold and condense out and still end up in the cold box with the possibility of explosive NOX gum formation if PSA recovery of ethylene from FCC Fuel gas was practiced as described in U.S. Pat. No. 5,245,099.
Accordingly, it is desirable to provide apparatuses and processes for recovery of ethylene from FCC absorber off-gas in a safer and a cost-effective manner. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.